This invention relates to a pneumatic servomotor used for assisting in the braking of a motor vehicle.
More particularly, the present invention relates to a pneumatic servomotor for an assisted braking of a motor vehicle, of the type comprising a rigid casing, in which a transverse partition wall is movable, thus defining in an airtight manner a front chamber under a first engine negative pressure, and a rear chamber under a second pressure, varying between the engine negative pressure and the atmospheric pressure; of the type including a moving piston, integral with the moving partition wall and travelling with the latter, and a control rod, moving inside the piston, in a selective manner as a function of an axial input force, exerted in the forward direction against a return force applied to the rod by a return spring; of the type according to which the control rod is biased towards either an intermediate actuation position or an end actuation position, resulting from the applying of the input force at a determined high speed; of the type comprising a plunger, arranged at the front part of the control rod inside the piston, and a three-way valve, including at least an annular seat borne by a rear section of the plunger, and capable of varying the second pressure, prevailing within the rear chamber, particularly by connecting the front chamber with the rear chamber when the control rod is in the rest position, or by gradually connecting the rear chamber with the atmospheric pressure when the control rod is actuated; of the type in which, in the end actuation position of the control rod, a rear face of a finger, which is slidingly fitted at the front end of the plunger, is biased by the plunger so that a front face of the finger may come into contact with a reaction disk, integral with the moving piston, so as to transmit the reaction force of the moving piston to the plunger and to the control rod; and of the type comprising a unidirectional clutch device, which includes at least one coaxial tubular sleeve, sliding on the plunger and the front end of which comprises the finger, and a latching element, movable between an active position, in which it does not cooperate with the sleeve, and an active position, controlled by the end actuation position of the control rod and in which it cooperates with a locking element of the sleeve, so as to lock the latter in an end front axial position in such a way that the finger may lock the moving piston, independently of the plunger and of the control rod.
In a well-known manner, such a design is most suitable as regards safety, in the case of an emergency braking situation.
As a matter of fact, a conventional servomotor comprises neither a finger nor a unidirectional clutch device for the finger. Thus the plunger is likely to bias directly the reaction disk, integral with the rear face of the moving piston.
In a full-braking situation, in which case a maximum braking force is applied on the control rod, the actuation of the control rod causes the finger-forming plunger to be actuated, which fact results in the maximum opening of the three-way valve and, therefore, the rear chamber is subjected to the atmospheric pressure. Thus, the moving partition wall travels forward and the end of the plunger contacts the reaction disk, integral with the rear face of the moving piston.
Therefore, the force, which is applied onto the moving piston when the control rod reaches the end of its stroke, results from the assisting force, arising from the pressure difference between each side of the moving partition wall, and from the force exerted by the finger-forming plunger onto said moving piston. Besides, the driver feels the braking reaction force, which is transmitted from the moving piston to the plunger, through the reaction disk.
As a matter of fact, it has been established that quite a number of drivers, when confronted with an emergency braking situation, underestimated the risks actually incurred and, after having jammed the brakes on, would somewhat release the braking force at the very time when a considerable force should have been maintained in order to avoid an accident.
In the case of a full-braking situation, accompanied by the swift travel of the control rod, the plunger may touch the reaction disk and therefore give the driver the feeling of a maximum braking action even before the pressure difference between the front and rear chambers actually reaches its maximum value, which may lead the driver to release the braking force even though it should be maintained so as to profit by the maximum braking force.
A servomotor, like that of the above-described type, makes it possible to eliminate such a disadvantage, in that the finger is locked into contact with the reaction disk, through the fixing of the sleeve, which results in a maximum force being maintained on the rear face of the moving piston, even though the driver may have released the braking force in part.
Besides, a unidirectional clutch device for such a servomotor has a substantially low manufacturing cost, since the finger is combined with the sleeve.
Yet, the servomotor of the previously described type has the disadvantage of requiring the compliance with accurate manufacturing dimensions for the sleeve.
As a matter of fact, in such a servomotor, the sleeve incorporating the finger is mounted in such a way that, in the rest position, a given clearance may be extant between the finger and the reaction disk.
The size of the clearance determines the assisting force actually supplied by the moving piston and for which a reaction force is transmitted, through the reaction disk, from the moving piston to the control rod. The magnitude of said assisting force is commonly called the xe2x80x9cjumpxe2x80x9d of the servomotor and, as a result, it depends on the finger dimensions and, in particular, on the finger thickness.
Moreover, in such a servomotor, if the input force happens to be applied onto the control rod at a higher speed than the determined speed, the axial position of the locking element of the sleeve conditions the speed at which the unidirectional clutch device is likely to start.
The speed is commonly known as the xe2x80x9ctripping speedxe2x80x9d and, therefore, it depends on given dimensions of the sleeve, more particularly the axial distance between the front end of the plunger and the locking element of the sleeve.
Conventionally, the locking element of the sleeve is a shoulder-forming transverse face, thus resulting in a sleeve which is suitable for a sound performance of the servomotor, when the sleeve is manufactured using a machining process, more especially a lathe shaping one. In the latter case, the dimensional requirements are met by the process itself.
Such design has the disadvantage of heavily burdening the cost price of the sleeve, since accurate dimensions must be complied with.
In order to cope with this difficulty, the present invention provides a sleeve, the dimensions of which can be adjusted merely by a plastic deformation process.
Therefore, this invention provides a servomotor of the above-described type, characterised in that the sleeve comprises a first adjusting part, situated between the front and rear faces of the finger, and a second adjusting part, situated between the rear face of the finger and the locking element, said parts being plastically deformable at least in the axial direction so as to allow the adjustment of the servomotor prior to the assembly.
According to other features of this invention:
the latching element consists of a substantially annular key, which surrounds the sleeve with a given clearance and which is capable of being driven by the moving piston, when the input force is applied, at the determined speed, so as to rock about a generally transverse axis, in such a way that a peg on the key may abut against a rear transverse face of the sleeve, constituting the locking element;
the sleeve comprises a tubular front cylindrical section, the end of which constitutes the finger, and a tubular rear cylindrical section having a substantially greater diameter than that of the front section, and including a radial groove, a shoulder-forming front transverse face of which is the locking rear transverse face;
the front section comprises an inner annular flange extending rearwardly and protruding from the rear face of the finger, said flange being axially deformable prior to the assembly so as to constitute the first adjusting part for the adjustment of the position, or jump of the servomotor, in which the finger will bias the reaction disk;
the front section and the rear section of the sleeve are separated by a reduced-thickness intermediate section, which is axially deformable so as to constitute the second adjusting part for the adjustment of the tripping position, in which the key peg abuts against the locking rear transverse face;
the groove exhibits a substantially truncated-cone-shaped profile, rearwardly of the shoulder-forming front transverse face;
the peg has the shape of a concave truncated-cone-shaped angular sector, which is complementary to the truncated-cone-shaped profile of the groove in the sleeve;
the key has, in an axial sectional view, the shape of a tee, the vertical branch of which is substantially radially directed and traversed by the sleeve, whereas its horizontal branch, which is substantially axially directed, is received, without any axial clearance, between two opposite walls of a cavity traversing the piston, perpendicularly to its axis, so as to allow but a rocking motion of the key inside the cavity;
the front horizontal half-branch of the tee key has a face which radially faces the sleeve and from which the peg protrudes;
the tee key is resiliently biased against the rear transverse wall of the cavity by two compression springs, arranged between the front transverse wall of the cavity and blind holes for the centering of the tee key and made on both sides of the sleeve, in the vertical branch of the tee;
the sleeve is made of a material exhibiting a low elastic limit arid a high toughness.
The invention also provides a setup for the plastic deformation of a tubular sleeve intended for a pneumatic servomotor of the previously described type, and comprising, in the front-to-rear direction, a tubular front cylindrical section having a determined outer diameter and closed at its end by a front transverse wall from which a deformable annular coaxial flange, constituting a first adjusting part, extends towards the inner part of the sleeve, a deformable intermediate section with the same outer diameter as the front section and constituting a second adjusting part, and a tubular rear cylindrical section having a determined diameter, greater than that of the front section, and including at least one shoulder-forming rear transverse face.
To this end, this invention provides a setup of the above-described type, characterised in that it comprises a male die and a female die, which are tubular and coaxial, the sleeve being placed between them so as to be deformed in a single axial-compression deformation process, in the course of which the dies are axially pressed together for the simultaneous attainment of the predetermined required dimensions for the first and second adjusting parts.
According to other features of this setup:
the inner diameter of the tubular female die is greater than the outer diameter of the rear section of the sleeve, and the female die comprises a rear transverse wall from which a cylindrical bearing surface extends in the forward direction, and on which the sleeve is fitted in such a way that a front end of the cylindrical bearing surface may rest on the annular flange of the sleeve;
the male die includes a bore with a diameter corresponding to the outer diameter of the front section of the sleeve, and a front transverse bottom of which will support the front transverse wall of the front section of the sleeve;
the male die comprises a convex cylindrical bearing surface, protruding rearwards and designed to receive a first tubular adjusting shim having a determined length and arranged in the continuation of the tubular female die;
the setup comprises a pressing front tubular spacer to be fitted on the sleeve, axially into contact with the shoulder-forming transverse face of the sleeve, and a second rear tubular adjusting shim having a determined length, both of them being provided for a fitting inside the female die, between the cylindrical bearing surface of the female die and its inner cylindrical wall, prior to the axial-compression deformation process;
the male die is stationary and the female die is mounted for an axial sliding motion so as to be pressed towards the male die with a compressive force having a determined value.
Other features and advantages of the present invention will be apparent from the following detailed description, when taken in conjunction with the accompanying drawings.
FIG. 1 is a general view of a servomotor according to the state of the art;
FIG. 2 is a detail axial sectional view, showing a pneumatic servomotor for an assisted braking according to this invention, the unidirectional clutch device being in the inactive position;
FIG. 3 is a detail axial sectional view, showing a pneumatic servomotor for an assisted braking according to this invention, the unidirectional clutch device being in the active position;
FIG. 4 is an axial sectional view, showing a sleeve for the unidirectional clutch device of FIG. 1 or 2; and
FIG. 5 is an axial sectional view of a setup for the plastic deformation of a sleeve according to FIG. 3.